Lattice dynamics coupled to charge and spin degrees of freedom in the molecular dimer-Mott insulator \(\kappa\)-(BEDT-TTF)\(_{2}\)Cu[N(CN)\(_{2}\)]Cl

Inelastic neutron scattering measurements on the molecular dimer-Mott insulator \(\kappa\)-(BEDT-TTF)\(_{2}\)Cu[N(CN)\(_{2}\)]Cl reveal a phonon anomaly in a wide temperature range. Starting from \(T_{\rm ins}\sim50\)-\(60\) K where the charge gap opens, the low-lying optical phonon modes become overdamped upon cooling towards the antiferromagnetic ordering temperature \(T_\mathrm{N} = 27\) K, where also a ferroelectric ordering at \(T_{\rm FE} \approx T_{\rm N}\) occurs. Conversely, the phonon damping becomes small again when spins and charges are ordered below \(T_\mathrm{N}\), while no change of the lattice symmetry is observed across \(T_\mathrm{N}\) in neutron diffraction measurements. We assign the phonon anomalies to structural fluctuations coupled to charge and spin degrees of freedom in the BEDT-TTF molecules.